Hi,
I should correct myself. That's not a bug. Probably the reason why i have said that way, i thought you were using generic child variant. Because with VM - Custom Field, Stockable variant the AJAX updating only used to update "price" and "add to cart button" (depends on stock number).
The rest of the information (between parent and child) remain constant and untouched for all product attributes.
Showing the availability image would create confusion. Because the available of the parent will be taken into consideration and it will never be updated based on selected product variant. Simply because AJAX is used only to transform the add to cart button depends on the selected child product's stock info. If the child product is "out of stock", then people will keep seeing the parent product's availability image and it will confuse the buyer. All in all, when the "VM - Custom, stockable variants" is used, the availability image has been eliminated from product details page on purpose. It is the same in original virtuemart because it is logical to eliminate it to avoid confusion.
BUT - if you use "generic child variant" then what ever availability image you choose for child products, will be printed individually. You will be able to choose different availability images for parent product and each child product. Then you will keep seeing the availability images.
If you have a "stock" concern, then use "Generic Child Variant" custom field all the time. Generic Child Variant, is more professional way of usage. (you will be allowed to change: product picture, product desc., availability images, stock number etc.. for each attributes)
If you don't have "stock" concern, then use the "Product Variant" or "Image Type" custom fields.
Regards.